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Polypropylene (PP), also known as polypropene, is a
thermoplastic A thermoplastic, or thermosoftening plastic, is a plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular mass, molecular weight. The polyme ...
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, repeating subunits. Due to their ...

polymer
used in a wide variety of applications. It is produced via
chain-growth polymerizationChain-growth polymerization (American spelling) or chain-growth polymerisation (British spelling) is a polymerization technique where Unsaturated compound, unsaturated monomer molecules add onto the active site on a growing polymer chain one at a tim ...
from the
monomerA monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrical ...

monomer
propylene Propene, also known as propylene or methyl ethylene, is an unsaturated organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical ...
. Polypropylene belongs to the group of
polyolefin A polyolefin is a type of polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentac ...
s and is partially crystalline and non-polar. Its properties are similar to
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes, containers including bottles ...

polyethylene
, but it is slightly harder and more heat resistant. It is a white, mechanically rugged material and has a high chemical resistance. Bio-PP is the bio-based counterpart of polypropylene (PP). Polypropylene is the second-most widely produced commodity plastic (after
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes, containers including bottles ...

polyethylene
). In 2019, the global market for polypropylene was worth $126.03 billion. Revenues are expected to exceed US$145 billion by 2019. The sales of this material are forecast to grow at a rate of 5.8% per year until 2021.


History

Phillips Petroleum Phillips Petroleum Company was an American oil company incorporated in 1917 that expanded into petroleum refining, marketing and transportation, natural gas gathering and the chemicals sectors. It was Phillips Petroleum that first found oil in the ...
chemists J. Paul Hogan and Robert Banks first demonstrated the polymerization of propylene in 1951. The stereoselective polymerization to the isotactic was discovered by
Giulio Natta Giulio Natta (26 February 1903 – 2 May 1979) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian language, a ...
and Karl Rehn in March 1954. This pioneering discovery led to large-scale commercial production of isotactic polypropylene by the Italian firm Montecatini from 1957 onwards. Syndiotactic polypropylene was also first synthesized by Natta.


Chemical and physical properties

of polypropylene Polypropylene is in many aspects similar to
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes, containers including bottles ...

polyethylene
, especially in Dissolution (chemistry)#Polymers, solution behaviour and electrical properties. The
methyl group A methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms — CH3. In chemical formula, formulas, the group is often skeletal formula#Pseudoelement symbols, abbreviated ...

methyl group
improves mechanical properties and thermal resistance, although the chemical resistance decreases. The properties of polypropylene depend on the molecular weight and molecular weight distribution, crystallinity, type and proportion of comonomer (if used) and the iso
tacticity. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The ...
. In isotactic polypropylene, for example, the methyl groups are oriented on one side of the carbon backbone. This
arrangement In music Music is the art of arranging sounds in time to produce a composition through the elements of melody, harmony, rhythm, and timbre. It is one of the universal cultural Culture () is an umbrella term which encompasses the s ...

arrangement
creates a greater degree of crystallinity and results in a stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene.


Mechanical properties

The density of (PP) is between 0.895 and 0.92 g/cm3. Therefore, PP is the commodity plastic with the lowest density. With lower density, with lower weight and more parts of a certain mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density. However, the density of polyethylene can significantly change with fillers. The
Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied lengt ...
of PP is between 1300 and 1800 N/mm². Polypropylene is normally tough and flexible, especially when
copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained by copolymerization of two monomer species are sometimes c ...
ized with
ethylene Ethylene (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science Co ...

ethylene
. This allows polypropylene to be used as an
engineering plastic Engineering plastics are a group of plastic Plastics are a wide range of synthetic polymers, synthetic or semi-synthetic materials that use polymers as a main ingredient. Their Plasticity (physics), plasticity makes it possible for plastics to b ...
, competing with materials such as
acrylonitrile butadiene styrene Acrylonitrile butadiene styrene (ABS) (chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and gener ...
(ABS). Polypropylene is reasonably economical. Polypropylene has good resistance to
fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
.


Thermal properties

The melting point of ''polypropylene'' occurs in a range, so the melting point is determined by finding the highest temperature of a
differential scanning calorimetry Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic ...

differential scanning calorimetry
chart. Perfectly isotactic PP has a melting point of . Commercial isotactic PP has a melting point that ranges from , depending on
atactic. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistrys. Stereochemistry focuses on stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomeri ...
material and crystallinity.
Syndiotactic. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The ...

Syndiotactic
PP with a crystallinity of 30% has a melting point of . Below 0 °C, PP becomes brittle. The thermal expansion of PP is very large, but somewhat less than that of polyethylene.


Chemical properties

Polypropylene at room temperature is resistant to fats and almost all organic
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
s, apart from strong oxidants. Non-oxidizing acids and bases can be stored in
container box. File:Railroad car with container loads.jpg, A Flatcar#Spine car, spine car with a tank container and an open-top intermodal container, intermodal shipping container with canvas cover. A container is any receptacle or enclosure for holding ...

container
s made of PP. At elevated temperature, PP can be dissolved in nonpolar solvents such as
xylene Xylene (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million ...
,
tetralin Tetralin (1,2,3,4-tetrahydronaphthalene) is a hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that conta ...

tetralin
and
decalin Decalin (decahydronaphthalene, also known as bicyclo[4.4.0]decane and sometimes decaline), a bicyclic organic compound, is an industrial solvent. A colorless liquid with an aromatic odor, it is used as a solvent for many resins or fuel additives. It ...
. Due to the tertiary carbon atom PP is chemically less resistant than PE (see Markovnikov rule). Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Isotactic & atactic polypropylene is soluble in ''p''-xylene at 140 °C. Isotactic precipitates when the solution is cooled to 25 °C and atactic portion remains soluble in ''p''-xylene. The melt flow rate (MFR) or melt flow index (MFI) is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow-molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease. There are three general types of polypropylene: homopolymer, random copolymer, and block copolymer. The comonomer is typically used with
ethylene Ethylene (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science Co ...

ethylene
. Ethylene-propylene rubber or EPDM added to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity, lowers the melting point and makes the polymer more transparent.


Molecular structure – tacticity

Polypropylene can be categorized as atactic polypropylene (PP-at), syndiotactic polypropylene (PP-st) and isotactic polypropylene (PP-it). In case of atactic polypropylene, the methyl group (-CH3) is randomly aligned, alternating (alternating) for syndiotactic polypropylene and evenly for isotactic polypropylene. This has an impact on the crystallinity (amorphous or semi-crystalline) and the thermal properties (expressed as glass transition point Tg and melting point Tm). The term
tacticity. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The ...
describes for polypropylene how the methyl group is oriented in the polymer chain. Commercial polypropylene is usually isotactic. This article therefore always refers to isotactic polypropylene, unless stated otherwise. The tacticity is usually indicated in percent, using the isotactic index (according to DIN 16774). The index is measured by determining the fraction of the polymer insoluble in boiling heptane. Commercially available polypropylenes usually have an isotactic index between 85 and 95%. The tacticity effects the polymers Physical property, physical properties. As the
methyl group A methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms — CH3. In chemical formula, formulas, the group is often skeletal formula#Pseudoelement symbols, abbreviated ...

methyl group
is in isotactic propylene consistently located at the same side, it forces the macromolecule in a Helix, helical shape, as also found in starch. An isotactic structure leads to a semi-crystalline polymer. The higher the isotacticity (the isotactic fraction), the greater the crystallinity, and thus also the softening point, rigidity, e-modulus and hardness. Atactic polypropylene, on the other hand, lacks any regularity which makes it unable to crystallize and Amorphous solid, amorphous.


Crystal structure of polypropylene

Isotactic polypropylene has a high Crystallization of polymers#Degree of crystallinity, degree of crystallinity, in industrial products 30–60%. Syndiotactic polypropylene is slightly less crystalline, atactic PP is Amorphous solid, amorphous (not crystalline).


= Isotactic polypropylene (iPP)

= Isotactic polypropylene can exist in various crystalline modifications which differ by the molecular arrangement of the polymer chains. The crystalline modifications are categorized into the α-, β- and γ-modification as well as mesomorphic (smectic) forms. The α-modification is predominant in iPP. Such crystals are built from lamellae in the form of folded chains. A characteristic anomaly is that the lamellae are arranged in the so-called "cross-hatched" structure. The melting point of α-crystalline regions is given as 185 to 220 °C, the density as 0.936 to 0.946 g·cm−3. The β-modification is in comparison somewhat less ordered, as a result of which it forms faster and has a lower melting point of 170 to 200 °C. The formation of the β-modification can be promoted by nucleating agents, suitable temperatures and shear stress. The γ-modification is hardly formed under the conditions used in industry and is poorly understood. The mesophase, mesomorphic modification, however, occurs often in industrial processing, since the plastic is usually cooled quickly. The degree of order of the mesomorphic phase ranges between the crystalline and the amorphous phase, its density is with 0.916 g·cm−3 comparatively. The mesomorphic phase is considered as cause for the transparency in rapidly cooled films (due to low order and small crystallites).


= Syndiotactic polypropylene (sPP)

= Syndiotactic polypropylene was discovered much later than isotactic PP and could only be prepared by using metallocene catalysts. Syndiotactic PP has a lower melting point, with 161 to 186 °C, depending on the degree of tacticity.


= Atactic polypropylene (aPP)

= Atactic polypropylene is amorphous and has therefore no crystal structure. Due to its lack of crystallinity, it is readily soluble even at moderate temperatures, which allows to separate it as by-product from isotactic polypropylene by Extraction (chemistry), extraction. However, the aPP obtained this way is not completely amorphous but can still contain 15% crystalline parts. Atactic polypropylene can also be produced selectively using metallocene catalysts, atactic polypropylene produced this way has a considerably higher molecular weight. Atactic polypropylene has lower density, melting point and softening temperature than the crystalline types and is tacky and rubber-like at room temperature. It is a colorless, cloudy material and can be used between −15 and +120 °C. Atactic polypropylene is used as a sealant, as an Insulating materials, insulating material for automobiles and as an additive to Asphalt, bitumen.


Copolymers

Polypropylene
copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained by copolymerization of two monomer species are sometimes c ...
s are in use as well. A particularly important one is polypropylene random copolymer (PPR or PP-R), a random copolymer with
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes, containers including bottles ...

polyethylene
used for plastic pipework.


PP-RCT

Polypropylene random crystallinity temperature (PP-RCT), also used for plastic pipework, is a new form of this plastic. It achieves higher strength at high temperature by β-crystallization of polymers, crystallization.


Degradation

Polypropylene is liable to chain degradation from exposure to temperatures above 100 °C. Oxidation usually occurs at the tertiary carbon centers leading to chain breaking via reaction with oxygen. In external applications, degradation is evidenced by cracks and crazing. It may be protected by the use of various polymer stabilizers, including UV-absorbing additives and anti-oxidants such as phosphites (e.g. tris(2,4-di-tert-butylphenyl)phosphite) and hindered phenols, which prevent polymer degradation. Microbial communities isolated from soil samples mixed with starch have been shown to be capable of degrading polypropylene. Polypropylene has been reported to degrade while in the human body as implantable mesh devices. The degraded material forms a tree bark-like layer at the surface of mesh fibers.


Optical properties

PP can be made translucent when uncolored but is not as readily made transparent as polystyrene, acrylic glass, acrylic, or certain other plastics. It is often Opacity (optics), opaque or colored using pigments.


Production

Polypropylene is produced by the
chain-growth polymerizationChain-growth polymerization (American spelling) or chain-growth polymerisation (British spelling) is a polymerization technique where Unsaturated compound, unsaturated monomer molecules add onto the active site on a growing polymer chain one at a tim ...
of propene: The industrial production processes can be grouped into gas phase polymerization, bulk polymerization and slurry polymerization. All state-of-the-art processes use either gas-phase or bulk reactor systems. *In gas-phase and slurry-reactors, the polymer is formed around heterogeneous catalyst particles. The gas-phase polymerization is carried out in a fluidized bed reactor, propene is passed over a bed containing the Heterogeneous catalysis, heterogeneous (solid) catalyst and the formed polymer is separated as a fine powder and then converted into Pelletizing, pellets. Unreacted gas is recycled and fed back into the reactor. *In bulk polymerization, liquid propene acts as a solvent to prevent the precipitation of the polymer. The polymerization proceeds at 60 to 80 °C and 30–40 atm are applied to keep the propene in the liquid state. For the bulk polymerization, typically Chemical reactor#CSTR (continuous stirred-tank reactor), loop reactors are applied. The bulk polymerization is limited to a maximum of 5% ethene as comonomer due to a limited solubility of the polymer in the liquid propene. *In the slurry polymerization, typically C4–C6 alkanes (butane, pentane or hexane) are utilized as Chemically inert, inert diluent to Suspension (chemistry), suspend the growing polymer particles. Propene is introduced into the mixture as a gas. The properties of PP are strongly affected by its
tacticity. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The ...
, the orientation of the
methyl group A methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms — CH3. In chemical formula, formulas, the group is often skeletal formula#Pseudoelement symbols, abbreviated ...

methyl group
s () relative to the methyl groups in neighboring monomer units (see #Molecular structure – tacticity, above). The tacticity of polypropylene can be chosen by the choice of an appropriate catalyst.


Catalysts

The properties of PP are strongly affected by its
tacticity. Tacticity (from Greek τακτικός ''taktikos'' "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The ...
, the orientation of the
methyl group A methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms — CH3. In chemical formula, formulas, the group is often skeletal formula#Pseudoelement symbols, abbreviated ...

methyl group
s ( in the figure) relative to the methyl groups in neighboring monomer units. A Ziegler–Natta catalyst is able to restrict linking of monomer molecules to a specific orientation, either isotactic, when all methyl groups are positioned at the same side with respect to the backbone of the polymer chain, or syndiotactic, when the positions of the methyl groups alternate. Commercially available isotactic polypropylene is made with two types of Ziegler-Natta catalysts. The first group of the catalysts encompasses solid (mostly supported) catalysts and certain types of soluble metallocene catalysts. Such isotactic macromolecules coil into a Helix, helical shape; these helices then line up next to one another to form the crystals that give commercial isotactic polypropylene many of its desirable properties. Another type of metallocene catalysts produce syndiotactic polypropylene. These macromolecules also coil into helices (of a different type) and crystallize. Atactic polypropylene is an amorphous rubbery material. It can be produced commercially either with a special type of supported Ziegler-Natta catalyst or with some metallocene catalysts. Modern supported Ziegler-Natta catalysts developed for the polymerization of propylene and other 1-alkenes to isotactic polymers usually use as an active ingredient and as a support. The catalysts also contain organic modifiers, either aromatic acid esters and diesters or ethers. These catalysts are activated with special cocatalysts containing an organoaluminum compound such as Al(C2H5)3 and the second type of a modifier. The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl2 and depending on the type of organic modifiers employed during catalyst preparation and use in polymerization reactions. Two most important technological characteristics of all the supported catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–80 °C under standard polymerization conditions. Commercial synthesis of isotactic polypropylene is usually carried out either in the medium of liquid propylene or in gas-phase reactors. Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts. They employ bridged bis-metallocene complexes of the type bridge-(Cp1)(Cp2)ZrCl2 where the first Cp ligand is the cyclopentadienyl group, the second Cp ligand is the fluorenyl group, and the bridge between the two Cp ligands is -CH2-CH2-, >SiMe2, or >SiPh2. These complexes are converted to polymerization catalysts by activating them with a special organoaluminum cocatalyst, methylaluminoxane (MAO).


Manufacturing from polypropylene

Melting process of polypropylene can be achieved via extrusion and Molding (process), molding. Common extrusion methods include production of melt-blown and spun-bond fibers to form long rolls for future conversion into a wide range of useful products, such as face masks, filters, diapers and wipes. The most common shaping technique is injection molding, which is used for parts such as cups, cutlery, vials, caps, containers, housewares, and automotive parts such as batteries. The related techniques of blow molding and injection-stretch blow molding are also used, which involve both extrusion and molding. The large number of end-use applications for polypropylene are often possible because of the ability to tailor grades with specific molecular properties and additives during its manufacture. For example, antistatic additives can be added to help polypropylene surfaces resist dust and dirt. Many physical finishing techniques can also be used on polypropylene, such as machining. Surface finishing, Surface treatments can be applied to polypropylene parts in order to promote adhesion of printing ink and paints. Expanded Polypropylene (EPP) has been produced through both solid and melt state processing. EPP is manufactured using melt processing with either chemical or physical blowing agents. Expansion of PP in solid state, due to its highly crystalline structure, has not been successful. In this regard, two novel strategies were developed for expansion of PP. It was observed that PP can be expanded to make EPP through controlling its crystalline structure or through blending with other polymers.


Biaxially oriented polypropylene (BOPP)

When polypropylene film is extruded and stretched in both the machine direction and across machine direction it is called ''biaxially oriented polypropylene''. Two methods are widely used for producing BOPP films, namely, the tenter process and tubular process. Biaxial orientation increases strength and clarity. BOPP is widely used as a packaging material for packaging products such as snack foods, fresh produce and confectionery. It is easy to coat, print and laminate to give the required appearance and properties for use as a packaging material. This process is normally called converting. It is normally produced in large rolls which are slit on Roll slitting, slitting machines into smaller rolls for use on packaging machines. BOPP is also used for stickers and labels, including by commercial sellers such as Sticker Mule. It is non-reactive which makes BOPP suitable for safe use in the pharmaceutical and food industry. It is one of the most important commercial polyolefin films. BOPP films are available in different thicknesses and widths. They are transparent and flexible.


Applications

box, with a living hinge and the resin identification code under its flap As polypropylene is resistant to fatigue, most plastic living hinges, such as those on flip-top bottles, are made from this material. However, it is important to ensure that chain molecules are oriented across the hinge to maximise strength. Polypropylene is used in the manufacturing of piping systems, both ones concerned with high purity and ones designed for strength and rigidity (e.g., those intended for use in potable plumbing, hydronic heating and cooling, and reclaimed water). This material is often chosen for its resistance to corrosion and chemical leaching, its resilience against most forms of physical damage, including impact and freezing, its environmental benefits, and its ability to be joined by heat fusion rather than gluing. Image:Red Polypropylene Chair with Stainless Steel Structure.JPG, left, upA polypropylene chair Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Its heat resistance also enables it to be used as the manufacturing material of consumer-grade kettles. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminum foil (both heat-resistant materials). After the product has cooled, the tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to polypropylene of the same thickness is readily apparent. Rugged, translucent, reusable plastic containers made in a wide variety of shapes and sizes for consumers from various companies such as Rubbermaid and Sterilite are commonly made of polypropylene, although the lids are often made of somewhat more flexible LDPE so they can snap onto the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered, or similar consumer products, although HDPE and polyethylene terephthalate are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, pharmacy prescription bottles, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature. A diversity of medical devices are made from PP. A common application for polypropylene is as biaxially oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear Plastic bag, bags. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products. Polypropylene, highly colorfast, is widely used in manufacturing carpets, rugs and mats to be used at home. Polypropylene is widely used in ropes, distinctive because they are light enough to float in water. For equal mass and construction, polypropylene rope is similar in strength to polyester rope. Polypropylene costs less than most other synthetic fibers. Polypropylene is also used as an alternative to polyvinyl chloride (PVC) as insulation for electrical cables for Low Smoke Zero Halogen, LSZH cable in low-ventilation environments, primarily tunnels. This is because it emits less smoke and no toxic halogens, which may lead to production of acid in high-temperature conditions. Polypropylene is also used in particular roofing membranes as the waterproofing top layer of single-ply systems as opposed to modified-bit systems. Polypropylene is most commonly used for plastic moldings, wherein it is injected into a mold while molten, forming complex shapes at relatively low cost and high volume; examples include bottle tops, bottles, and fittings. It can also be produced in sheet form, widely used for the production of stationery folders, packaging, and storage boxes. The wide color range, durability, low cost, and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in Rubik's Cube stickers because of these characteristics. The availability of sheet polypropylene has provided an opportunity for the use of the material by designers. The light-weight, durable, and colorful plastic makes an ideal medium for the creation of light shades, and a number of designs have been developed using interlocking sections to create elaborate designs. Polypropylene sheets are a popular choice for trading card collectors; these come with pockets (nine for standard-size cards) for the cards to be inserted and are used to protect their condition and are meant to be stored in a binder. Expanded polypropylene (EPP) is a Polymeric foam, foam form of polypropylene. EPP has very good impact characteristics due to its low stiffness; this allows EPP to resume its shape after impacts. EPP is extensively used in model aircraft and other radio controlled vehicles by hobbyists. This is mainly due to its ability to absorb impacts, making this an ideal material for RC aircraft for beginners and amateurs. Polypropylene is used in the manufacture of loudspeaker drive units. Its use was pioneered by engineers at the BBC and the patent rights subsequently purchased by Mission Electronics for use in their Mission Freedom Loudspeaker and Mission 737 Renaissance loudspeaker. Polypropylene fibres are used as a concrete additive to increase strength and reduce cracking and spalling. In some areas susceptible to earthquakes (e.g., California), PP fibers are added with soils to improve the soil's strength and damping when constructing the foundation of structures such as buildings, bridges, etc. Polypropylene fibres are also used in drywall joint compound for reinforcement. It can increase the flexibility and dimensional stability of the joint compound and reduce shrinkage and cracking when it dries. Polypropylene is used in polypropylene drums. In June 2016, a study showed that a mixture of polypropylene and durable Oleophobic, superoleophobic surfaces created by two engineers from Ohio State University can repel liquids such as shampoo and oil. This technology could make it easier to remove all of the liquid contents from polypropylene bottles, particularly those that have high surface tension such as shampoo or oil.


Clothing

Polypropylene is a major polymer used in nonwovens, with over 50% used for diapers or sanitary products where it is treated to absorb water (hydrophilic) rather than naturally repelling water (hydrophobic). Other non-woven uses include filters for air, gas, and liquids in which the fibers can be formed into sheets or webs that can be pleated to form cartridges or layers that filter in various efficiencies in the 0.5 to 30 micrometre range. Such applications occur in houses as water filters or in air-conditioning-type filters. The high surface-area and naturally oleophilic polypropylene nonwovens are ideal absorbers of oil spills with the familiar floating barriers near oil spills on rivers. Polypropylene, or 'polypro', has been used for the fabrication of cold-weather base layers, such as long-sleeve shirts or long underwear. Polypropylene is also used in warm-weather clothing, in which it transports sweat away from the skin. Polyester has replaced polypropylene in these applications in the U.S. military, such as in the Extended Cold Weather Clothing System, ECWCS. Although polypropylene clothes are not easily flammable, they can melt, which may result in severe burns if the wearer is involved in an explosion or fire of any kind. Polypropylene undergarments are known for retaining body odors which are then difficult to remove. The current generation of polyester does not have this disadvantage. Some fashion designers have adapted polypropylene to construct jewelry and other fashion accessory, wearable items.


Medical

Its most common medical use is in the synthetic, nonabsorbable surgical suture, suture Prolene, manufactured by Ethicon Inc. Polypropylene has been used in hernia and pelvic organ prolapse repair operations to protect the body from new hernias in the same location. A small patch of the material is placed over the spot of the hernia, below the skin, and is painless and rarely, if ever, rejected by the body. However, a polypropylene mesh will erode the tissue surrounding it over the uncertain period from days to years. A notable application was as a transvaginal mesh, used to treat vaginal prolapse and concurrent urinary incontinence. Due to the above-mentioned propensity for polypropylene mesh to erode the tissue surrounding it, the FDA has issued several warnings on the use of polypropylene mesh medical kits for certain applications in pelvic organ prolapse, specifically when introduced in close proximity to the vaginal wall due to a continued increase in number of mesh-driven tissue erosions reported by patients over the past few years. On 3 January 2012, the FDA ordered 35 manufacturers of these mesh products to study the side effects of these devices. Due to the outbreak of the COVID-19 pandemic in 2020, the demand for PP has increased significantly because it's a vital raw material for producing melt blowing, meltblown fabric, which is in turn the raw material for producing facial masks. for pulse applications with metal foil manufactured by WIMA.


Niche

Very thin sheets (≈2–20 µm) of polypropylene are used as a dielectric within certain high-performance Pulsed power, pulse and low-loss Radio frequency, RF capacitors. Expanded polypropylene (EPP) Polymeric foam, foam is a structural material in hobbyist radio control model aircraft. Unlike expanded polystyrene foam (EPS) which is friable and breaks easily on impact, EPP foam is able to absorb kinetic impacts very well without breaking, retains its original shape, and exhibits memory form characteristics which allow it to return to its original shape in a short amount of time. When the cathedral on Tenerife, La Laguna Cathedral, was repaired in 2002–2014, it turned out that the vaults and dome were in a rather bad condition. Therefore, these parts of the building were demolished, and replaced by constructions in polypropylene. This was reported as the first time this material was used in this scale in buildings. Under the trade name Ulstron polypropylene rope is used to manufacture scoop nets for whitebait. It has also been used for sheets of yacht sails. Polymer banknotes are made from BOPP, where it provides a durable base and allows for the use of transparent security features by omitting opaque inks in the desired areas.


Recycling

Polypropylene is recyclable and has the number "5" as its resin identification code: File:Symbol_Resin_Code_5_PP.svg, 50px However, only about 1% of all polypropylene in the world is actually recycled.


Repairing

Many objects are made with polypropylene precisely because it is resilient and resistant to most solvents and glues. Also, there are very few glues available specifically for gluing PP. However, solid PP objects not subject to undue flexing can be satisfactorily joined with a Epoxy resin, two-part epoxy glue or using hot-glue guns. Preparation is important and it is often helpful to roughen the surface with a file, emery paper or other abrasive material to provide better anchorage for the glue. Also it is recommended to clean with mineral spirits or similar alcohol prior to gluing to remove any oils or other contamination. Some experimentation may be required. There are also some industrial glues available for PP, but these can be difficult to find, especially in a retail store. PP can be melted using a speed tip welding technique. With speed welding, the plastic welder, similar to a soldering iron in appearance and wattage, is fitted with a feed tube for the plastic weld rod. The speed tip heats the rod and the substrate, while at the same time it presses the molten weld rod into position. A bead of softened plastic is laid into the joint, and the parts and weld rod fuse. With polypropylene, the melted welding rod must be "mixed" with the semi-melted base material being fabricated or repaired. A speed tip "gun" is essentially a soldering iron with a broad, flat tip that can be used to melt the weld joint and filler material to create a bond.


Health concerns

The advocacy organization Environmental Working Group classifies PP as of low to moderate hazard. PP is Wet processing engineering#Solution dyeing, dope-dyed; no water is used in its dyeing, in contrast with cotton. In 2020 researchers reported that polypropylene Baby bottle, infant feeding bottles with contemporary preparation procedures were found to cause microplastics#Humans, microplastics exposure to infants ranging from 14,600 to 4,550,000 particles per capita per day in 48 regions. Microplastics release is higher with warmer liquids and similar with other polypropylene products such as lunchboxes.


Combustibility

Like all organic compounds, polypropylene is combustible. The flash point of a typical composition is 260 °C; autoignition temperature is 388 °C.


References


External links


Chain structure of PolypropylenePolypropylene on PlastipediaPolypropylene Properties & other information
{{Authority control Dielectrics Packaging materials Plastics Polyolefins Thermoplastics Commodity chemicals